Tsinghua University team achieves 100 km quantum direct communication for the first time in the world

Recently, the team of Professor Long Guilu of the Department of Physics and the team of Professor Lu Jianhua of the Department of Electronics have designed and implemented a new quantum direct communication system with mixed encoding of phase quantum states and timestamp quantum states. The communication distance reaches 100 kilometers, which is currently the longest in the world. Quantum direct communication distance. Such indicators can realize point-to-point quantum direct communication between cities without relays, and can support some applications of wide-area quantum networks based on secure classical relays.

In 2000, Professor Long Guilu proposed the first protocol for quantum direct communication. Quantum direct communication uses quantum state as a carrier to encode and transmit information, which is a new paradigm of quantum secure communication. Quantum direct communication develops reliable communication under noisy channels to reliable and secure communication under noisy and eavesdropping channels. In 2016, a joint team from Tsinghua University and Shanxi University completed the first experimental demonstration of quantum direct communication. In 2017, related teams from the University of Science and Technology of China, Nanjing University of Posts and Telecommunications, and Tsinghua University respectively completed demonstration experiments of entanglement-based quantum direct communication. In 2019, Professor Long Guilu's team and Professor Lu Jianhua's team cooperated to successfully develop the world's first quantum direct communication system, achieving a secure communication rate of 50bps at a 1.5km optical fiber distance. In 2020, they released the world's first practical quantum direct communication prototype at the Zhongguancun Forum, achieving a transmission rate of 4kbps in a 10-kilometer optical fiber; in the same year, they further increased the communication distance to 18km.

In previous systems, sampling detection and information transmission all use phase quantum states. The new system adopts the mixed encoding of phase quantum state and time stamp quantum state, and the time stamp quantum state is used for sampling detection, which greatly reduces the influence of noise. Communication still uses phase quantum states with self-compensating properties. Therefore, the new system has a high degree of stability and a very low intrinsic bit error rate (bit error rate when there is no eavesdropping). Combined with the extremely low bit rate LDBCH encoding with stronger error correction capability, it effectively improves the secure communication capacity and distance. and rate.

The new system increases the maximum tolerable loss from 5.1dB to 18.4dB under the 50MHZ laser pulse frequency, and the longest communication distance in commercial low-loss single-mode fiber reaches 100 kilometers, breaking the previous longest distance of 18 kilometers. The communication rate of the new system has also been improved, reaching 22.4kbps at a fiber distance of 30 kilometers. The new system still has a lot of room for improvement in laser pulse frequency, and the corresponding communication distance and speed are expected to be further improved to meet the application requirements of some scenarios.

The results were published in the top journal "Light-Science & Applications", with Zhang Haoran (mentor Academician Wang Chongyu) and Sun Zhen (mentor Academician Lu Jianhua) as the co-first authors. Professor Long Guilu of the Department of Physics and Researcher Yin Liuguo of the National Research Center for Information are the co-corresponding authors.

Link:

[1]https://doi.org/10.1038/s41377-022-00769-w

[2]https://www.phys.tsinghua.edu.cn/info/1178/5091.htm